Optical elements of the aforementioned type are usually manufactured from transparent materials, for example, glass, glass ceramics or plastic. In order to reduce or compensate optical aberrations, the structures having an optical effect needed to be produced in a highly precise fashion. Manufacturing methods with mechanical processing steps are known for this purpose, for example, the grinding and polishing of lenses or the production of gratings, in particular of blazed gratings, by forming grooves on an optical substrate.
In other pertinent manufacturing methods known from the state of the art, the structures having an optical effect are produced by moulding or hot embossing structures produced on a master or a tool on the surface of an optical substrate. In this case, the structures produced on the master correspond to a negative of the structures having an optical effect to be produced. Hot-forming or hot-embossing methods are employed for moulding or hot embossing the structures produced on the master on the surface of the optical substrate. In these methods, the optical substrate is heated to a temperature, at which its surface can be deformed, wherein the optical substrate and the master are pressed against one another with sufficient pressure for moulding or hot embossing the structures produced on the master on the surface of the optical substrate. In another known method, the material of the optical substrate is poured or injected into a mould, in which the master is suitably arranged. The latter-mentioned method is particularly suitable for the mass production of optical elements consisting of plastic materials.
One common aspect of all aforementioned methods is that the master and the optical substrate need to be separated from one another (demoulded). This limits the attainable aspect ratio, i.e., the depth-to-width ratio, of the structures to be realized, namely to values of approximately 1:1. Since the moulding or hot embossing process always takes place near or above the melting temperature of the material of the optical substrate, it is unavoidable that the material of the optical substrate tends to adhere to the master during the demoulding process, i.e., when the master is lifted off the optical substrate. This lowers the accuracy of the moulding or hot embossing process and adversely affects the service life of the master and its precision. In this respect, it needs to be taken into account that the advantages of the aforementioned moulding techniques are only cost-effective if a large quantity of optical elements can be manufactured with constant precision by means of the same master without requiring costly subsequent processing steps. The reason for this can be seen, in particular, in the comparatively high costs for manufacturing precise masters and hot-forming or hot-embossing tools.
It is known to provide the surface of the master with a protective coating in order to prevent the material of the master from directly adhering to the optical substrate. Such a protective coating needs to fulfill strict requirements. The protective coating, in particular, needs to be connected to the substrate of the master in a rigidly adhering fashion, wherein the wettability of the protective coating with the material of the optical substrate should also be low. This significantly restricts the selection of materials for the protective coating, as well as the coating techniques for coating the master substrate. In addition, a highly homogenous and true-to-contour coating needs to be applied on the structured surface of the master with the chosen coating technique such that the coated structures on the surface of the master can be moulded or hot embossed on the substrate precisely. This additionally restricts the selection of materials for the protective coating and of the coating technique to be used.
The structures can be produced very precisely on the surface of a master substrate with techniques known from the manufacture of semiconductor components, particularly photolithography techniques. However, the protective coating leads to a certain distortion of the structures, particularly to the rounding of edges and to surface roughness. These effects need to be taken into account, in particular, when moulding very fine structures and/or structures with high aspect ratios.
Consequently, it would be desirable to develop precise and durable masters or tools for use in hot-forming and hot-embossing methods. Since the masters are used, in particular, for the mass production of optical elements, significant economical advantages can also be attained in the mass production of optical elements by means of costly manufacturing methods.
A method for embossing a waveguide in a deformable gel layer at room temperature is disclosed in “Embossing techniques for fabricating integrated optical components in hard inorganic wave guiding materials” by W. Lukosz et al., Optical Letters, October 1983, Vol. 8, No. 10, pp. 537-539. The thin film is produced from organometallic compounds by means of a dip-drawing method. After the embossing, the film is hardened at temperatures of several 100° C. and transformed into an inorganic oxide material. During the embossing, a substrate provided with the organic sol-gel thin layer is pressed against a master grating. The master grating is coated with an aluminum layer.
U.S. Pat. No. 6,591,636 B1 discloses a tool and a method for forming glass. Oxidation and corrosion processes were observed during the glass forming, particularly in the forming of glass compounds with significant alkali components and/or alkaliferous components. This leads, in particular, to undesirable dull glass surfaces. Different oxidation-resistant and corrosion-resistant compounds are suggested for coating the forming tool, wherein said compounds are applied by means of electroplating or physical vapor deposition (PVD) or chemical vapor deposition (CVD).
JP 2003-342025 A discloses a master for manufacturing finely structured optical elements, for example, microlens arrays, gratings, Fresnel zoned lenses and the like. The accuracy of the structures lies below one micrometer. The structures are initially produced by means of a photolithographic process on a dummy by means of synchrotron radiation. A nickel-based alloy is applied on the thusly structured dummy by means of electroplating. After the dummy is lifted off the coating, a master for manufacturing the optical elements is obtained.